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The globular cluster Terzan 5 is a fossil of the early Milky Way

The globular cluster Terzan 5 seen by the MAD on the VLT (Image ESO/F. Ferraro)

An article published in “The Astrophysical Journal” describes a research on the globular cluster Terzan 5. An international team of astronomers led by Francesco Ferraro from the University of Bologna discovered that the stars of Terzan 5 are divided into two groups, one with an age of 12 billion years and one with an age of around 4.5 billion years, more or less like the Sun. This unique feature can help to better understand the evolution of the Milky Way.

The globular cluster Terzan 5 is one of those discovered by Agop Terzan. Also cataloged as IRC-20385, is about 19,000 light years from Earth, in the central region of the Milky Way. This type of cluster has a more or less spherical shape and is composed of a large number of stars with very strong gravitational bonds, in particular in its central area, hence its shape.

This research derives from the discovery in 2009 that the cluster Terzan 5 contained two different populations of stars whose chemical characteristics were different. It took years to establish the age of the two populations and this helped to better understand the mechanisms of star formation within it and that it’s a kind of fossil that dates back to the birth of the Milky Way.

Astronomers used for their research data collected by the various telescopes’ instruments: the Multi-conjugate Adaptive Optics Demonstrator (MAD), a prototype equipped with an adaptive optics system installed on ESO’s Very Large Telescope (VLT), the Hubble space telescope’s Wide Field Camera 3 (WFC3) and the Keck Observatory’s Near-Infrared Camera 2 (NIRC2).

The presence of two populations of stars of such different ages indicates that Terzan 5 ancestor contained a considerable amount of gas. After the birth of the first generation of stars it retained enough of that gas to form the second generation. The estimate is that there was a mass of at least 100 million solar masses to create the second generation of stars.

Terzan 5 is a kind of fossil dating back to the early stages of the formation of the Milky Way and was probably one of the first components of the galaxy core. In fact, according to the current models of formation, these cores form from the merging of different gas and stars clumps. Certain characteristics of Terzan 5 are very similar to those of the clumps observed in very distant galaxies where star-forming processes are being observed.

Generally, the various clumps merge so completely that their traces get lost. A globular cluster such as Terzan 5 has actually preserved a certain identity within the galactic core, presumably because of the strong gravitational bonds existing within it. It’s a stroke of luck because it provides us with information about the birth of the Milky Way and the galaxy formation processes in general.